The exceptional properties of heavy water as a neutron moderator make it useful in nuclear reactors and in particular the CANDU nuclear reactor developed by Atomic Energy of Canada Limited. Most of the world's heavy water supplies are currently provided by the Girdler-Sulphide process or processes based on ammonia-hydrogen catalytic exchange. The Girdler-Sulphide process is a bithermal (two temperature) heavy water production process and takes advantage of differences in thermodynamic separation factors between water and hydrogen sulphide. The process uses a cascaded series of dual-temperature, mass transfer columns circulating large quantities of hydrogen sulphide gas countercurrently to a water feed. Both the Girdler-Sulphide process and ammonia-hydrogen catalytic exchange require large capital expenditures. The ammonia process has size limitations and the Girdler-Sulphide process consumes large amounts of energy and utilizes very hazardous hydrogen sulphide.
The high cost of heavy water produced using the Girdler-Sulphide process and ammonia-based processes can affect the economic attractiveness of heavy-water moderated reactors such as CANDU. Accordingly, a number of alternative processes have been proposed for heavy water production. Among them are processes that exploit deuterium isotope exchange between water and hydrogen using a catalyst.
One such process is known as Combined Electrolysis and Catalytic Exchange (“CECE”). The CECE process has previously been described in U.S. Pat. No. 3,974,048. The CECE heavy water production process is a monothermal process that extracts heavy water from normal water by a combination of electrolysis and catalytic exchange between the water feeding electrolytic cells and the hydrogen produced in them. The primary components of a normal multi-stage CECE process are each stage's hydrogen water catalytic exchange enrichment columns, oxygen-stream vapour scrubber columns and electrolytic cells. The catalytic exchange columns enrich water flowing down the column by stripping deuterium from the up-flowing hydrogen gas, with conditions always favouring deuterium transfer to the liquid. Electrolytic cells provide a bottom reflux flow by converting the enriched liquid leaving the catalytic exchange column into hydrogen gas. The electrolytic cells in a CECE process not only provide a bottom reflux flow but also enrich the cell liquid inventory. Because the entire feed stream must be electrolysed, the cost of electrolysis can result in a prohibitively expensive process for heavy water extraction and is practical only as a parasitic process where large scale electrolysis is performed for other reasons.
A second process, Combined Industrially Reforming and Catalytic Exchange (“CIRCE”), is a parasitic monothermal process. CIRCE uses an industrial monothermal steam reformer for the first stage to generate hydrogen from methane and water feeds and electrolysis (typically CECE) for higher stages. Although it is more complex than the CECE process, the main attraction of the CIRCE process is the widespread availability of relatively large plants producing hydrogen by steam reforming. The CIRCE process suffers from the fact that elevated levels of deuterium in the reformer mean that leaks of any deuterated species (water, methane, hydrogen) from the reformer are particularly costly and the plant requires a high level of leak tightness in the reformer. Optimization of the CIRCE process for the lowest unit cost is primarily a balance between minimizing first stage catalyst volume and loss of deuterium with reformer leakage (by moving separative work into the higher stages) and minimizing electrolytic cell capital costs (by moving separative work into the first stage).
An alternative process to harness water-hydrogen exchange is the Bithermal Hydrogen Water (“BHW”) process. BHW is a non-parasitic process using liquid phase catalytic exchange (“LPCE”) to generate heavy water. In each stage there is an upper cold tower where the deuterium transfers from the hydrogen to the liquid water, and a lower hot tower where the deuterium transfers from the water to the hydrogen gas. The feed to the higher stages is taken from between the cold and hot towers. The BHW process is similar to the Girdler-Sulphide process, but with the advantages of much superior separation factors, lower energy consumption and non-toxic and non-corrosive process fluids. BHW liquid phase catalytic exchange stages can advantageously be substituted for most or all of the CECE upper stages of the CIRCE process. Such a hybrid system can result in a process that is more cost effective than a conventional CIRCE process.
The CECE, CIRCE and BHW processes rely upon wet-proofed catalysts to catalyze the exchange reaction between the hydrogen gas and the liquid water. The preferred catalyst is a Group VIII metal (most preferably platinum) having a liquid-water repellent organic polymer or resin coating thereon selected from the group consisting if polyfluorocarbons (preferably polytetrafluoroethylene), hydrophobic hydrocarbon polymers of medium to high molecular weight, and Silicones, and which are permeable to water vapour and hydrogen gas. Catalysts of this nature are described in U.S. Pat. Nos. 3,981,976, 4,126,667 and 4,471,014. LPCE implements highly active structured catalytic modules, which incorporate hydrophobic catalytic layers and hydrophilic mass-transfer layers. Isotope exchange occurs on the hydrophobic catalytic layers while mass transfer occurs between water vapour and liquid water within the catalyst module.
Platinum is widely accepted as the most active catalyst at high and low temperatures and other comparable conditions for hydrogen isotope exchange compared to other metals used as catalysts. The cost of catalysts used in LPCE processes represents a significant fraction of the entire heavy water production cost. Platinum presents a significant contribution to the overall cost of the structured catalyst. With the price of platinum steadily increasing, a clear cost advantage can be achieved by reducing the dependency on platinum for hydrogen isotope exchange catalysis.
Highly active, low-cost catalysts for hydrogen isotope exchange between hydrogen and water have been a long-term objective internationally since the 1950s. Researchers have explored either non-Pt based catalysts or catalysts containing Pt at a significantly reduced loading in combination with other metals.
Thus, there remains a need for a highly active and highly stable catalyst that is less costly than conventional mono-platinum catalysts for heavy water production and other isotope exchange processes between water and hydrogen.